1. Technical Field
The present invention relates to a magneto-optical disc with magnetic field modulation system.
2. Background of the Invention
Optical discs as mass information carrier media are now attractive. Among these, there is a magneto-optical magnetic field modulation system that can be overwritten and so is expected to be applied to data files, etc. According to this magnetic field modulation procedure, the magnetic layer of a magneto-optical disc is irradiated with a laser beam from an optical pickup in a DC manner to increase its temperature and, at the same time, a modulated magnetic field is applied onto the magnetic layer through a magnetic head located on the side opposite to the optical pickup for overwriting recording.
So far, this recording has been mainly achieved by the CSS method in which the disc starts to rotate while in contact with the magnetic head and returns to the state of contact upon stopping. A magnetic head of the type that is constantly in contact with the disc has been proposed as well. For that reason, it is required for this type of magneto-optical disc that the surface protective film located on the magnetic head side be protected against adsorption and have some durability with respect to head crash, etc.
Various proposals have been made for such protective films. For instance, JP-A-2-301040 and JP-A-2-301041 (laid open for public inspection) propose the use of a resin composition composed mainly of a polyurethane type resin containing fluorine, and JP-A-3-37844 (laid open for public inspection) comes up with the use of a resin composition containing a fluoroplastic soluble in an organic solvent as the main component. It has also been proposed to incorporate in these resin compositions a metal element larger in ionization tendency than the transition metal element forming a recording film or an filler for lubrication.
In addition, JP-A-2-40149 (laid open for public inspection) proposes the incorporation of a lubricant in an ultraviolet curable resin, and JP-A-3-17844 (laid open for public inspection) uses a lubricant layer. JP-A-3-62338 (laid open for public inspection) makes a proposal to the effect that an overcoat layer is provided on its surface with uniform undulations of 0.1 .mu.m to 0.5 .mu.m inclusive.
However, these are all less than satisfactory. For instance, a smooth surface formed of resin alone is insufficient in terms of lubrication, and the incorporation of metals or fillers for lubrication, if they are hard, poses a problem that the associated head is damaged. The use of a lubricant is unpreferable as well in terms of disc properties.